While fused silica is known for its brittleness on macroscopic scale [1], it exhibits an amount of plasticity on microscale [2]. Thermally-treated Stöber-Fink-Bohn (SFB)-type silica spheres are known to approach the structure of vitreous silica and show size-dependent mechanical properties [3,4]. Adequate electron-beam (e-beam) irradiation can be used to induce enormous ductility during compression of nanoscale silica spheres [5-7], and to alter their Young’s modulus (E) [7]. While a controlled introduction of structural anisotropy by cooling of glass melts under load [8-10] was shown to enhance the mechanical properties of glass fibers [11], we recently showed that e-beam-assisted quenching under load (turning off the e-beam during compression) inside the transmission electron microscope (TEM) may also lead to structural anisotropy and affects the mechanical properties of nanoscale silica spheres [12]. Here we prove the potential of e-beam-assisted quenching under load on fused silica pillars and further investigate their size-dependent mechanical behavior.

Fused silica pillars are prepared by two different methods from bulk fused silica, namely (1) reactive ion etching (RIE) and (2) focused ion beam (FIB) milling in combination with a charge neutralizer system (FEI Company). Mechanical testing was performed with the Hysitron PI95 TEM Picoindenter^TM in the TEM and a custom-built indenter in the scanning electron microscope (SEM) [6]. Both, RIE and FIB milling lead to pillar structures with reproducible geometry and suitable for in situ mechanical experiments in TEM and SEM (Fig. 1). In situ compression of RIE pillars to high strains in the SEM eventually results in fracture with characteristic star-like fracture pattern (Fig. 2). In situ compression experiments at smaller strains carried out on FIB-prepared pillars in the TEM at beam-off conditions reveal a fully elastic deformation behavior, as exemplarily shown in Fig. 3. Thereby, an E = 78 GPa and compressive strength of ≥ 8 GPa are achieved. While E is slightly higher, the compressive strength clearly exceeds the one known for bulk fused silica [1], and the one of microscale fused silica pillars [2]. Further compression experiments on pillars in the TEM and SEM are planned, with the aim to explore their overall size-dependent mechanical behavior in direct relation to our work on nanoscale glass spheres [4,7]. Finally, we expand our recently reported e-beam-assisted quenching under load approach [12] also on fused silica pillars, with the aim to get a generalized picture of the mechanical properties of nanoscale glasses upon quenching under load in the TEM.

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[12] M. Mačković et al., submitted.
Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the SPP1594 “Topological Engineering of Ultra-Strong Glasses”, Cluster of Excellence EXC 315 “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged.

Figures:

Fig. 1. Fused silica pillars prepared by RIE (top) and FIB (bottom): a) - c) are SEM images and d) a TEM image.

Fig. 2. In situ mechanical testing of fused silica pillars (prepared by RIE) in the SEM: SEM images in a) and b) show the setup before and after compression, respectively. In c) a top view SEM image of a fractured fused silica pillar is shown.

Fig. 3. In situ compression experiment of a FIB-prepared fused silica pillar at beam-off conditions in the TEM: a) mechanical data; b) and c) are TEM images before and after in situ compression in the TEM, respectively.

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-compression-experiments-of-fused-silica-pillars-in-the-tem-and-sem/